The present invention relates to a process for producing melamine-formaldehyde microcapsules and to the microcapsules produced thereby. More particularly, the present invention relates to an improved process for producing melamine-formaldehyde microcapsules using a protective colloid which has been coupled with a melamine-formaldehyde pre-condensate.
In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated back side is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front side is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff precursor then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording paper is well know.
Also well known are self-contained sheets which have the CB coating and the CF coating layered or admixed on a support sheet. Such sheets are also considered carbonless copy papers.
Microencapsulation has been used in the production of carbonless copy papers for some time. It is well know to use melamine-formaldehyde (sometimes hereinafter referred to as "MF") in the microencapsulation process as the material out of which the microcapsule wall is constructed. Typically, a water soluble MF pre-condensate is dissolved in an aqueous solution (known as the external phase). A discontinuous phase of a material to be encapsulated (known as the internal phase or core material) is emulsified in the external phase using a water soluble, surface active polymer as a protective colloid. Generally, the internal phase will consist of droplets of an oily solution. In the production of carbonless copy paper, the internal phase wall contain therein a dissolved dyestuff precursor solution.
A condensation reaction of the MF pre-condensate is next initiated by lowering the pH of the emulsion. As the molecular weight of the MF pre-condensate increases, it precipitates (or more precisely liquid-liquid phase separates) onto the oil droplets whereon further condensation and cross-linking of the MF and MF pre-condensate completes the formation of the capsule wall.
Variations of this general reaction scheme can be found in the prior art. For example, Kureha, U.S. Pat. Nos. 4,460,722 and 4,562,116 disclose a complex coacervation reaction in which a cationic urea resin and a MF pre-condensate simultaneously condense in the presence of an anionic surfactant.
Regardless of the method of MF microencapsulation chosen, the most critical step is that of getting the phase-separated MF polymer to collect uniformly on and around the internal phase droplets without destabilizing the emulsion. The success or failure of this step is dependent upon the choice of protective colloid used to establish the initial emulsion.
There is an inherent contradiction in the role of the protective colloid. On one hand, the protective colloid stabilizes the emulsion by orienting itself at the internal phase/external phase interface, thus establishing a steric and/or charged boundary layer around each droplet. This layer serves as a barrier to other particles or droplets preventing their intimate contact and coalescence and thereby maintains uniform droplet size. On the other hand, the protective colloid must aid, or at least allow, phase-separated MF polymer particles to pass freely to and collect at the internal phase/external phase interface. Failure to find an adequate solution to this contradiction in demands on the protective colloid results in thickening or gelling of the microcapsule slurry, formation of aggregates of microcapsules instead of single microcapsule droplets, and non-uniform microcapsule sizes.
One solution disclosed in the prior art exploits the weakly cationic (partially positive charge) nature of the MF condensate at low ph. Anionic (negatively charged) protective colloids are used to stabilize the emulsion. For example, Sliwka, U.S. Pat. No. 4,406,816, discloses the use of a water soluble homopolymer or copolymer having sulfonic acid groups attached thereto as an anionic protective colloid. Similarly, Mitsui, U.S. Pat. No. 4,574,110, teaches the use of an acrylic copolymer as an anionic protective colloid. The negatively charged protective colloid layer about the droplet is no barrier to the oppositely charged MF condensate particles. As the MF continues to polymerize and grow, it physically displaces the protective colloid from the internal phase/external phase interface and begins to form the microcapsule wall.
However, the MF pre-condensate is also positively charged at low pH and is therefore similarly attracted to and associates with the negatively charged protective colloid. This association destabilizes the emulsion by reducing the charged layer about the droplets when the pH is lowered to begin the self-condensation reaction of the MF pre-condensate. The destabilized emulsion allows internal phase droplets to combine with other internal phase droplets to form larger droplets. These large droplets in turn become large, non-uniformly sized microcapsules.
Another problem that occurs as a result of the association between the MF pre-condensate and protective colloid is the formation of aggregates of microcapsules due to the fact that the protective colloid is unable to separate the microcapsules as the phase-separated MF is polymerizing and cross-linking around the internal phase droplets. When these semi-liquid microcapsules come into contact with each other, polymerizing MF in the microcapsule walls cross-links with polymerizing MF in other microcapsule walls and thereby forms large conglomerates of microcapsules.
To overcome these problems, steps must be implemented to minimize the destabilization induced by the association between the MF pre-condensate and the anionic protective colloid. Hoshi et. al., U.S. Pat. No. 4,409,156, for example, discloses the use of a polyvalent isocyanate to stabilize the emulsion. However, such prior art methods are difficult to carry out and therefore must be meticulously monitored and manipulated by highly skilled operators. To maintain uniform droplet size, high speed mixing is required. However, the high speed mixing must be stopped precisely when wall formation begins or the freshly condensed MF polymer will be sheared from the internal phase droplet without forming a wall thereon.
In addition, the reaction rate must be strictly controlled through pH and temperature changes to maintain sufficient time intervals between the emulsification and capsule wall formation steps to allow the protective colloid to dissociate from the polymerizing MF pre-condensate to re-establish a steric boundary around the internal phase droplets. The strict monitoring and precise reaction condition manipulations restrict the prior art encapsulation processes to a batch operation. Even so, the successful production of uniformly sized MF microcapsules is difficult to obtain.
Accordingly, the need exists in the art for an improved MF microencapsulation process which is easier to carry out and results in the production of more uniformly sized microcapsules.